1. Field of the Invention
The present invention relates to flow meters, and in particular, to the housing configuration of variable area flow meters for measuring the flow rate of a fluid such as respiratory gas in a conduit.
2. The Prior Art
Long term monitoring of respiratory air flow in critical care patients, and in patients undergoing anesthesia, is essential for correctly assessing the patient's condition, and for selecting the course of future treatment. Of course, under these circumstances, because of the necessity of very accurate and reliable measurements, it is important that the flow meter which is selected meet all of the requirements for proper operation under critical care conditions. In particular, such operating conditions require use of a lightweight flow meter having a small dead space, a broad pressure range, and an accuracy that is not affected by the presence of fluid, including mucus produced by the patient.
There are a number of different types of flow meters that are well known in the technology and which are acceptable for short term use in applications such as diagnostic pulmonary measurement. These flow meters typically comprise a housing which may be connected through a pair of sensing ports to a differential pressure transducer. The housing is connected in-line to a conduit or respiratory line which conducts gas to and from the patient, for breathing purposes.
In general, the prior devices operate by creating a resistance across a flow of gas and then using the differential pressure transducer to measure the two differing pressures communicated through the sensing ports located on either side of the resistance. This pressure differential is mathematically related to a quantity of flow by the transducer. The resistance to the gas flow is often embodied in a membrane having an orifice which is smaller than the conduit. It is well known that during laminar flow conditions the relationship between flow and pressure differential is linear. However, depending on the type of resistance employed, turbulent flow may develop locally and thus, significantly distort the linear relationship. Consequently, variable size orifices have been employed to minimize the effects of turbulent flow on the differential pressure measurements.
One example of a prior art device that incorporates a variable area orifice is described in U.S. Pat. No. 4,083,245 issued to Osborn. In the Osborn device, the orifice is created by the movement of a hinged flap which opens in the direction of flow, thus, increasing the orifice area as the flow in the conduit increases. The flap is cut out of a membrane which is installed across the opening of the interior of the device housing.
Prior devices also typically include a membrane housing such as that disclosed in the Osborn reference, whose configuration prevents the free-flow of liquids which enter the housing from the connected conduit. In particular, moisture or mucus communicated from the patient can accumulate either in a depressed well surrounding the lower portion of the membrane, or up against a membrane ridge which protrudes from the invert of the housing. The accumulated liquids are unsanitary and, moreover, prevent effective operation of the flap, thereby reducing the accuracy of the sensing device. Furthermore, removal of the accumulated liquids requires that the respiratory line be frequently shut down in order to remove the flow meter for cleaning or replacement.
In view of the above, it would be an important improvement in the art to provide a variable orifice flow meter that prevents substantially all liquids that enter the line from accumulating within the meter, and hence maintains measurement accuracy. It would be a further improvement in the art to provide a variable orifice flow meter which enhances safety to the patient by eliminating unsanitary buildup of liquids in the meter while also minimizing the downtime required to service or replace the flow meter.